(1) Field of the Invention
The present invention relates to a method and apparatus for controlling the supply of fuel into an internal combustion engine. More particularly, the present invention relates to a fuel supply control method and apparatus of a sequential injection system, in which fuel supply values disposed for respective cylinders are independently actuated at predetermined supply-initiating times and in each cylinder, the supply of a fuel is effected at a time matched to the intake stroke of the cylinder.
(2) Description of the Related Art
As an example of the conventional method and apparatus for controlling the supply of fuel according to a sequential injection system, there is the method and apparatus disclosed in Japanese Unexamined Patent Publication No. 57-8328. The fuel supply control of this sequential injection system is advantageous in that in each cylinder, an air-fuel mixture in which air and fuel are sufficiently mixed can be supplied, there being no dispersion among the cylinders, and the variation of torque can be reduced.
However, the fuel supply control of this sequential injection system involves a problem in that at the transient driving state, a detection delay is caused in an air flow meter for detecting the quantity of intake air or a pressure sensor for detecting the intake pressure or delay of the computation of the fuel supply quantity by the apparatus, and the flow rate of intake air or the intake pressure is changed during the period from the point of the final setting of the fuel supply quantity to the intake stroke where the supplied fuel is practically inputted. Accordingly, for example for acceleration, the fuel supply quantity is set at a level lower than the level corresponding to the actual intake air quantity or intake pressure, and hence, the air-fuel ratio of the intake air-fuel mixture becomes excessively lean and the concentration of hydrocarbon HC or nitrogen oxide NO.sub.x in the exhaust gas is increased or a delay of the response of the average effective pressure by misfire by a lean air-fuel ratio is caused, with the result that an acceleration shock is caused or the acceleration response characteristic is degraded.
With this background, we previously proposed a fuel supply control apparatus in which the change of the state of intake air for transient driving is estimated and the fuel supply quantity is corrected based on the estimated change to improve the precision, of the fuel supply control at the transient driving state (see Japanese Unexamined Patent Publication No. 1-2237333 and U.S. patent application Ser. No. 261,887, now U.S. Pat. No. 4,947,816, patented Aug. 14, 1990).
In correcting the fuel supply quantity based on the estimated change of the state of intake air at the transient driving state, the quantity of the change of the engine load is computed based on the opening of the throttle valve and the engine revolution number, the detection of which is not delayed, and the time from the present point to the point of the predetermined crank angle position at the intake stroke (at the target crank angle position for the fuel supply control) is determined, it is estimated from the present change quantity how the engine load will change until the predetermined crank angle position of the intake stroke where the air-fuel mixture is actually inputted, and while regarding the estimated quantity of the change as corresponding to the excess or shortage of the fuel supply quantity, the normal fuel supply quantity sequentially controlled based on the intake air flow quantity Q or intake pressure PB is corrected.
In the case where the time Xms from the present point to the target crank angle position during the intake stroke (preferably the time at which the intake force is largest during the intake stroke), which is data necessary for the above-mentioned correction control, is increased as the time up to the target crank angle position during the intake stroke in the cylinder where the injection is now to be initiated, every time a reference angle signal REF emitted in the vicinity of the position of the initiation of the sequential injection of each cylinder (#1 to #4) from a crank angle sensor is received, and this time Xms is reduced by a unit time with the lapse of time until receipt of the subsequent reference angle signal REF, that is, in the case where one timer is changed over to a different cylinder every time the reference angle signal REF, for example, if the initial detection of the change of the engine load is performed in the state where the above-mentioned time Xms is changed over to the data of cylinder #3 and the target crank angle position of cylinder #1 where the injection is effected before cylinder #3 is not yet reached, the interrupt injection quantity is set based on the time up to the target crank angle position of cylinder #3 and the ratio of the change of the engine load, and it is impossible to perform the interrupt injection corresponding to the change of the load after the normal injection with respect to cylinder #1.
In the case where the fuel correction value is set based on the above-mentioned time Xms and the ratio of the change of the load of the engine and the fuel quantity for sequential injection is corrected by this fuel correction quantity, supposing that the ratio of the change of the engine load is computed at every 10 milliseconds, if this compution time of 10 milliseconds is just before the sequential injection and the time Xms is changed over to that for the cylinder where the injection of the fuel is going to be performed, the desired correction is made. However, if the compution time comes while the time Xms corresponding to the cylinder where the sequential injection is effected and there is no chance of the computation before the initiation of the next sequential injection, the correction quantity for the cylinder of the previous injection is used for the present sequential injection and the desired correction cannot be made.
In this case, if the period of the reference angle signal REF is added to the time Xms, even if the time Xms is not changed over to that corresponding to the cylinder of the next sequential injection, the time from the present point to the target crank angle position of the cylinder of the next sequential injection can be set, but if the revolution number increases and there is no chance of the computation of the correction quantity during the interval of the initiation of the sequential injection, even if the above-mentioned correction of the time Xms is made, the desired correction cannot be attained.